Dev Browser

An MCP (Model Context Protocol) server that lets OpenCode (and other MCP clients) drive a real Chrome/Chromium tab locally by sending Chrome DevTools Protocol (CDP) commands—without requiring Chrome to be launched with --remote-debugging-port.

It solves: “I want an agent to click/type/navigate/screenshot an existing local browser session” using a lightweight extension bridge + local relay.

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Architecture overview

This project runs an MCP server over stdio. Under the hood it talks to a local relay that connects to a Chrome/Chromium extension.

Extension-mode relay + endpoints

The relay exposes:

• ws://HOST:PORT/extension
  The browser extension connects here (one connection at a time).

• ws://HOST:PORT/cdp
  A CDP-like WebSocket endpoint. The MCP server connects here and sends CDP JSON messages.

Tools like dev_browser_goto, dev_browser_click, etc. ultimately send CDP commands (e.g. Page.navigate, Runtime.evaluate, Page.captureScreenshot) over /cdp. The relay forwards those commands to the extension, and forwards CDP events back to connected clients.

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Prerequisites

• Node.js >= 18
• Chrome/Chromium with the dev-browser extension installed (the extension must be running and able to connect to ws://HOST:PORT/extension)
• OpenCode installed and configured to run MCP servers

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Setup

npm install
npm run build

This produces dist/ and a runnable entrypoint.

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OpenCode configuration

OpenCode supports both a global config and a project config. In both cases, add an MCP server entry named dev_browser.

Global config: ~/.config/opencode/opencode.json

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "dev_browser": {
      "type": "local",
      "command": ["node", "/Users/<you>/Code/devtools/dev-browser-mcp/dist/index.js"],
      "enabled": true,
      "environment": {
        "HOST": "127.0.0.1",
        "PORT": "9222",
        "RELAY_MODE": "auto"
      }
    }
  }
}

Project config: ./opencode.json

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "dev_browser": {
      "type": "local",
      "command": ["node", "./dist/index.js"],
      "enabled": true,
      "environment": {
        "HOST": "127.0.0.1",
        "PORT": "9222",
        "RELAY_MODE": "auto"
      }
    }
  }
}

Notes:

• No secrets/tokens are required.
• The MCP server communicates over stdio; OpenCode will launch it via the command array.

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Environment variables

• HOST (default: 127.0.0.1)
  Host for the relay HTTP/WS server.

• PORT (default: 9222)
  Port for the relay HTTP/WS server.

• RELAY_MODE (default: auto)
  Controls whether the MCP server starts or attaches to the relay:

  • auto: probe http://HOST:PORT/ first; if a relay is already running, attach. Otherwise start a relay.
    • If starting the relay fails with EADDRINUSE, it probes http://HOST:PORT/ again (longer timeout). If the relay is reachable, it attaches; otherwise it rethrows the original error.
  • start: always start its own relay (will fail on port conflicts).
  • connect: never start; require an existing relay at http://HOST:PORT/ (throws if unreachable).

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Usage examples (tool calls)

Typical flow: ensure the extension is connected → open/select a tab → navigate → interact → snapshot/screenshot.

1) Wait for the extension to connect

{
  "tool": "dev_browser_ensure_extension_connected",
  "input": { "timeoutMs": 30000, "pollIntervalMs": 250 }
}

2) Open (or reuse) a named tab

{
  "tool": "dev_browser_page_open",
  "input": { "name": "my-tab" }
}

3) Navigate

{
  "tool": "dev_browser_goto",
  "input": {
    "url": "https://example.com",
    "waitUntil": "load",
    "timeoutMs": 30000
  }
}

4) Snapshot (AI-friendly DOM outline)

{
  "tool": "dev_browser_snapshot",
  "input": {}
}

The snapshot includes snapshotRef identifiers you can use for click/type.

5) Click / type

Click by CSS selector:

{
  "tool": "dev_browser_click",
  "input": { "selector": "button[type='submit']" }
}

Click by snapshotRef (from dev_browser_snapshot output):

{
  "tool": "dev_browser_click",
  "input": { "snapshotRef": "e123" }
}

Type by CSS selector:

{
  "tool": "dev_browser_type",
  "input": { "selector": "input[name='q']", "text": "hello", "clearFirst": true }
}

Type by snapshotRef:

{
  "tool": "dev_browser_type",
  "input": { "snapshotRef": "e456", "text": "hello", "clearFirst": true }
}

6) Screenshot

{
  "tool": "dev_browser_screenshot",
  "input": { "fullPage": false, "saveToFile": true }
}

If saveToFile=true (default), it writes a PNG under:

• ./.opencode/dev_browser/

…and returns the file path (plus the image content).

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Tool list

• dev_browser_relay_status — Get relay status (wsEndpoint, extensionConnected, mode); in auto it will start or attach.
• dev_browser_ensure_extension_connected — Wait until extensionConnected=true (polls relay /).
• dev_browser_pages_list — List named pages registered in the relay.
• dev_browser_page_open — Get or create a named tab via the relay and select it for subsequent actions.
• dev_browser_page_delete_mapping — Delete a named page mapping in the relay.
• dev_browser_goto — Navigate the selected tab to a URL.
• dev_browser_click — Click an element by CSS selector or snapshotRef (from dev_browser_snapshot).
• dev_browser_type — Type into an input by CSS selector or snapshotRef (from dev_browser_snapshot).
• dev_browser_wait_for_selector — Wait for a selector on the selected tab.
• dev_browser_evaluate — Evaluate JavaScript in the selected tab context.
• dev_browser_screenshot — Take a PNG screenshot; writes to ./.opencode/dev_browser and returns the file path.
• dev_browser_snapshot — Return an AI-friendly snapshot (YAML-ish) for the selected tab.

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Relationship to dev-browser (SawyerHood/dev-browser)

This project was built to expose the same core idea as SawyerHood/dev-browser to MCP clients (OpenCode): drive a real browser with persistent state and provide LLM-friendly DOM snapshots.

What we reuse / align with

• Extension-mode CDP relay concept: dev-browser has an “extension mode” where a local relay bridges CDP messages between an extension (chrome.debugger) and automation clients.
• Named pages: both systems expose a POST /pages abstraction to create/activate a named tab and return a targetId.
• Snapshot refs: dev-browser’s snapshot approach uses an injected script that returns a YAML-ish outline with stable [ref=eN] markers.

What’s different here

• Protocol surface: dev-browser is distributed as a Claude Code plugin/skill; this repo is a standalone MCP server designed for OpenCode.
• Control path: instead of shipping a “skill client” API, this exposes discrete MCP tools (dev_browser_goto, dev_browser_click, etc.) that send CDP commands.
• Relay ownership for multi-OpenCode: this MCP supports RELAY_MODE=auto so multiple OpenCode instances can attach to a single relay process without crashing on EADDRINUSE.
• Deterministic tab targeting: actions are scoped to a specific Chrome tab via targetId + Target.attachToTarget sessions so separate OpenCode instances can reliably control different named tabs.

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Troubleshooting

extensionConnected=false

• Ensure Chrome/Chromium is running and the dev-browser extension is installed/enabled.
• The extension must be able to connect to ws://HOST:PORT/extension.
• Run dev_browser_relay_status to confirm the relay is reachable and to see extensionConnected.

EADDRINUSE / port conflicts

• PORT=9222 is commonly used by other tooling (including Chrome remote debugging).
• Fix by choosing a different port:
  • set PORT (and keep it consistent for both relay + extension)
  • or set RELAY_MODE=connect if you intentionally run a relay elsewhere and want to attach.

Multiple OpenCode instances / tab naming

• Prefer explicit, unique tab names via dev_browser_page_open (e.g. "myproject-admin", "myproject-patient").
• If you don’t pass pageName to tools, they operate on the “last selected” page; avoid collisions by always opening/selecting a named page per workspace/agent.

Where screenshots are written

• By default, screenshots are saved to:
  • ./.opencode/dev_browser/
• If you don’t see files, confirm your current working directory (OpenCode project root) and that saveToFile=true.